Heh ! No problem with High Voltage, it's a buffer, so I'll not need a big transformer to get a high voltage, it's low current, right? I just want a cheap buffer that can sound well !

I'll will drive a BPA200 amplifier (LM3886 "200W" power amp), WITHOUT it's buffer. It have already a buffer in the manufactures schematic, because due the four CI's to be driven, the imput impedance is relatively low.

While we're waiting, I thought I'd just add my 2c. The second circuit, the cathode follower, has a much lower output impedance than the first circuit - maybe around 1 or 2k, depending on the tube. A cathode follower is useful to feed an interconnect cable.

What are you expecting from the tube buffer? If you want 'tube' sound SY herethical line stage (buffer) won't work - search the thread and look at the distortion figures... Than it may be better to use an ECC88 with low voltage - a la Musical Fidelity Cans. Or Joe Rasmussen's buffer with ECC88.

My idea is replace the buffer U5 in that attached schematic with the tube, so I go for non inverted. The gain can be one, like the U5, but if I can get a little more will be nice, two is fine!

The tube have to be the less expensive possible ! I shouldn't spend money on this project, I have others priorities heh ! BUT I'll spend some for sure ! Case, transformers and PCB are all already done and mounted! Today I'll buy the capacitors and resistors and put this thing to work !

I do not expect "sound tube" from the amplifier, I don't believe that chips can have this sound just because of a little tube in the middle of circuit... But, I expect more spaced sound, with less saturation ! Should I expect it? Not THAT sound, just some improvement.

The second schematic, the cathode follower, will be a better choice. I think impedance is even lower than 1K. It is some fraction of the cathode resistance, if memory serves. Cathode resistances are usually some 300 to 600 ohms (1/transconductance).

Anyway, in this instance, you will probably get a warmer sound, simply because it is a tube.

Then, because feedback is considered to be a little over 100%, you would technically get no distortion.

So, you can concievably replace your U5 with the tube. But use the follower.

A buffer is defined as an amplifier with no voltage gain, but some current gain. So the cathode follower can take a 3-4 volts input at 0.05 milliamps and put out a 3-4 volt signal at 1-2 mA. In the case of the cathode follower, it has a slightly negative gain, which means if you input 2 volts, the output might be 1.8 volts.

Now, I would suggest using a 12AU7 or 12AT7, since you can set them up with a higher current output. They match your situation better.

Gabe

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Gabe CGV Electronics
Home of the CGV-300B amplifier on a budget

The circuit in your post #8: You have a very high amplifier input impedance as a result of the input voltage follower, so why "destroy" that with a low 10K input resistor. More serious for me is the input electrolytic capacitor (unless you use a relatively expensive polypropylene). I do not tend to get frenetic about the use of electrolytics as some, but a polyester is better here, and there is no reason (at least that I can see) for not using e.g. a 100K/330nF or even 220K/150nF. Would you still need a buffer then?

Secondly the cathode follower is better as already said, but I am a little worried about the low (35V) anode supply voltage quoted. I did not calculate, but it looks suspiciously low to put you in the high 2nd harmonic distortion operating area. (OK, you have full negative feedback; still.) If voltage is not a problem, for triodes I would personally prefer that to be over 100V.